The present invention relates to a liquid ejecting apparatus such as an ink jet printer, and more particularly, to a liquid ejecting apparatus having a liquid ejecting head that introduces liquid reserved in a liquid reservoir into a pressure chamber through a liquid passage and that ejects the liquid introduced into the pressure chamber as droplets.
A representative example of a liquid ejecting apparatus having a liquid ejecting head that can eject liquid and ejecting various kinds of liquid from the liquid ejecting head can include an image forming apparatus such as an ink jet printer that performs a printing operation by ejecting and landing ink droplets onto a printing sheet as an ejection target (printing medium). In recent years, the liquid ejecting apparatus was not limited to the image forming apparatus, but was applied to various manufacturing apparatuses. In apparatuses for manufacturing displays such as liquid crystal displays, plasma displays, organic EL (electroluminescence) displays, and FEDs (surface emission displays), the liquid ejecting apparatus was used to eject color materials or various liquid materials for electrodes and the like to pixel forming areas or electrode forming areas.
A liquid ejecting apparatus that has a carriage mounted with the liquid ejecting head and a liquid reservoir (liquid source) and that ejects liquid in the liquid reservoir as droplets from the liquid ejecting head while moving the carriage was known (on-carriage type).
For example, like a business printer for performing a printing operation on a printing sheet having a large size, in a liquid ejecting apparatus using a relatively large amount of liquid once, a configuration in which a liquid source (ink cartridge) as a liquid reservoir is disposed in an apparatus body, a relay unit (ink pressure control unit which serves as a pressure control valve for controlling a pressure change at the time of supplying ink) for introducing ink from the liquid source into the liquid ejecting head is fitted to the liquid ejecting head, the liquid source and the relay unit are connected to each other through a flexible liquid supply tube, and ink is supplied from the liquid source to the liquid ejecting head through the liquid supply tube is employed (off-carriage type) (Patent Document 1).
In the configuration employing the ink cartridge representative of the liquid source, it is ideal that an ink passage (liquid passage) from an ink introduction needle inserted into the ink cartridge to a nozzle opening of a print head is filled with ink, but gas may permeate the ink passage due to replacement of the ink cartridge or the like, which it is difficult to completely prevent. Specifically, in an off-carriage type in which ink is supplied to the liquid ejecting head through a liquid supply tube from the ink cartridge, external air may permeate the wall surface of the liquid supply tube and may be melted in the ink, thereby saturating the ink in the ink passage. Accordingly, gas newly permeating the ink passage may be not melted in the ink, or the melted gas may get bubbles due to a change in temperature and may be mixed into the ink in the ink passage. The bubbles entering the ink passage in this way becomes greater gradually and when excessively grown bubbles moves to the pressure chamber by means of a flow of ink, pressure loss due to the bubbles absorbing pressure change at the time of performing an ejecting operation or lack in ink supply due to the bubbles' blocking the passage may be caused.
In order to prevent the problem with the mixture of bubbles, the liquid ejecting head is made to periodically perform a cleaning operation of generating a flow of ink at a flow rate greater by several times than that at the time of performing a printing operation, thereby discharging the bubbles in the ink passage. As a configuration for discharging the gas mixed into the ink other than the cleaning operation, a configuration in which a gas permeable film is disposed on a side surface of a common liquid chamber communicating with the pressure chamber, a chamber is disposed opposite a side of the gas permeable film contacting liquid, and the liquid in the common liquid chamber is degassed by generating a negative pressure in the chamber to prevent bubbles form being generated in the pressure chamber has been suggested (Patent Document 2).    Patent Document 1: Japanese Patent Publication No. 2005-219229A    Patent Document 2: Japanese Patent Publication No. 2006-95878A
However, in the configuration disclosed in the Patent Document 2, when a pressure difference between the common liquid chamber and the chamber is not great (for example, at least 50 kPa or more at the room temperature and preferably in the range of 80 kPa to 100 kPa), the gas in the common liquid chamber cannot permeate the gas permeable film to degas ink, thereby requiring a large-sized depressurizing means. In addition, when the pressure difference is too great, steam mixed in the ink gets out and the ink in the ink passage increases in viscosity. Accordingly, it is necessary to control the pressure in the chamber while monitoring the pressure with a pressure gauge. The gas permeable film formed of a fluorine-based film or a silicon-based film decreases in permeability due to permeation of the ink. When the ink is not sufficiently degassed in the common liquid chamber and bubbles are abruptly mixed into the ink passage due to attachment and detachment of the ink cartridge, the bubbles may enter the adjacent pressure chamber to cause a problem before the bubbles are melted in the ink.
The gas permeable film may not endure the pressure and be destroyed when the pressure difference increases, or the gas permeable film may be warped due to the pressure change and thus the ink may be carelessly ejected from the nozzle openings of the liquid ejecting head. The gas permeable film may absorb the pressure change at the time of ejecting the ink from the nozzle openings of the liquid ejecting head to cause the pressure loss, thereby badly influencing the ejection characteristic, such as a decrease in amount or rate of the ink ejected.
When it is intended to employ the gas permeable film, a film attaching process is necessary, thereby deteriorating the manufacturing efficiency. Since the rigidity of the film portion is small, supporting pillars (supporting members) for reinforcing the peripheral portion thereof are necessary. The provision of the supporting pillars may cause an increase in passage resistance of the passage, thereby badly influencing the ejection frequency and the like. In order to dispose the supporting pillars, it is necessary to consider the balance among the degassing efficiency, the head rigidity, and the passage resistance of the common liquid chamber, thereby making the design thereof difficult.